1. Field of the lnvention
The present invention relates to the preparation of crosslinked plastics by means of the isocyanate addition polymerization where hydroxyl group-containing compounds and polyisocyanates are foamed in molds in the presence of blowing agents and catalysts. More particularly, the invention relates to a process to improve the flowability of reactive polyurethane-polyurea systems which have adequate mechanical properties for the resulting molded articles.
2. Description of the Prior Art
The preparation of crosslinked plastics by means of the isocyanate addition polymerization process is known. According to Federal Republic of Germany AS No. 11 96 864 (U.S. Pat. No. 3,099,516), hydroxyl group-containing compounds and polyisocyanates are foamed in molds in the presence of blowing agents and catalysts.
By properly selecting the hydroxyl group-containing polyesters, polyethers, polyesteramides, etc. and the organic polyisocyanates, as well as by additionally using chain extenders such as glycols or diamines, this method may be used to produce elastic as well as rigid polyurethanes, as well as all intermediate modifications.
According to Federal Republic of Germany Pat. No. 831 604 (U.S. Pat. No. 2,778,810), isocyanate group-containing prepolymers are first prepared from the hydroxyl group-containing compounds and polyisocyanates in order to prepare the polyurethane elastomers. The prepolymers are then reacted in a second step with the chain extenders to form high molecular weight elastomers.
Generally, it has not been possible to use diamines as chain extenders in the one-shot process. According to Federal Republic of Germany AS No. 11 49 523 (U.S. Pat. No. 3,105,062), crystalline, aromatic primary diamines in amounts insufficient to saturate the isocyanate group are incorporated in the liquid, isocyanate group-containing prepolymers at a temperature below the melting point of the diamine. The mixtures are then cured by applying heat. According to Federal Republic of Germany Pat. No. 12 40 654 (U.S. Pat. No. 3,428,610), the isocyanate group-containing prepolymers are reacted at room temperature or at moderately elevated temperatures with those liquid or dissolved aromatic diamines which possess at least one linear alkyl substituent in the ortho position relative to the amino group and which possess two linear alkyl substituents having from 1 to 3 carbon atoms in the ortho position relative to the second amino group.
A process for the preparation of optionally cellular, elastic molded articles having a closed skin made of polyurethane-polyurea elastomers using the reaction injection molding (RIM) technique is described in Federal Republic of Germany AS No. 26 22 951, (U.S. Pat. No. 4,218,543). The cited systems are essentially comprised of organic polyisocyanates, polyols, reactive aromatic di- or polyamines which are substituted by alkyl groups in the ortho position relative to the amino group, and strong catalysts for the reaction between the hydroxyl and isocyanate groups. Here it is essential that the aromatic di- or polyamine be miscible in all proportions with polyols having molecular weights from 1800 to 12,000, and further that the alkyl substituents contain from 1 to 3 carbon atoms, whereby at least two of the alkyl substituents must contain 2 to 3 carbon atoms and each of the ortho positions relative to the amino qroups must be substituted. Such systems possess cream times down to less than one second. The transition from the liquid into the solid phase occurs almost instantly, which causes the liquid reaction mixture to solidify uniformly on the walls of the mold.
It is also known that the reactivity of aromatically bonded amino groups with respect to isocyanates can be reduced sharply by electrophilic substituents. Examples of such aromatic diamines are provided in Federal Republic of Germany Pat. No. 12 16 538 (GB Pat. No. 981,935): 3,3'-dichloro-4,4'-diaminodiphenylmethane, 3,3'-dinitro-4,4-diaminodiphenylmethane. Because of the associated health hazards, the processing of such substances is complex and expensive and is subject to burdensome regulations. However, the highly electro-negative substituents in these compounds lower the reactivity of the aromatically bonded amino groups so severely that it can require up to 15 minutes to fully cure parts produced by means of reaction molding, so that the use of such compounds is not economical.
Polyurethane-polyurea formulations having somewhat reduced reactivity compared with the systems described in Federal Republic of Germany AS No. 26 22 951 are obtained according to European patent application No. 26 915 when 3,3', 5,5'-tetraalkyl-substituted 4,4'-diamincdiphenylmethanes in which the alkyl radicals are identical or different and signify a methyl, ethyl, isopropyl, sec-butyl or tert-butyl radical, whereby at least one of the substituents must be an isopropyl or sec-butyl radical, are used as the aromatic diamines. The cited tetraalkyl-substituted diaminodiphenylmethanes are very easily miscible at room temperature with the required amounts of polyols. They exhibit little or no tendency to crystallize, so that these formulations are easily handled under conditions typical of conventional RIM systems. However, it is been found that the cited tetraalkyl-substituted 4,4'-diaminodiphenylmethanes may be too reactive for special applications.
Compared to European patent application No. 26 915, somewhat more reactive polyurethane-polyurea formulations are described in European patent application No. 69 286. Trialkyl-substituted meta-phenylenediamines are used as the aromatic diamines, whereby two of the alkyl-substituents must be identical or different and must be linear or branched alkyl radicals having from 1 to 4 carbon atoms and the third alkyl radical must possess from 4 to 12 carbon atoms or be a five- or six-membered cycloalky- radical. Even at relatively high diamine content, these formulations exhibit flowability and they produce molded articles having high dimensional stability at elevated temperatures and no progressive drop in shear modulus curves between 100.degree. and 200.degree. C.
The use of isocyanate group-containlng prepolymers and quasiprepolymers having isocyanate contents between 9 and 31 percent by weight, prepared by using optionally modified 4,4'-diphenylmethane diisocyanate, is known from U.S. Pat. Nos. 4,048,105; 4,102,833; and 4,374,210.
In the known processes, 4,4'-diphenylmethane diisocyanate or carbodiimide group-containing or urethane group-containing polyisocyanate mixtures based on 4,4'-diphenylmethane diisocyanate which are liquid at room temperature are used almost exclusively as the organic polyisocyanates, since molded parts produced in a similar manner using non-modified toluene diisocyanates did not exhibit sufficient green strength, cured too slowly, and did not have adequate mechanical properties. Efforts have been made to match the reactivity of the RIM systems to the requirements of the molds, for example its geometry, by selecting suitable substituted diamines and catalysts. The disadvantage of this method, though, is that the aromatic diamines which are used not only affect the reactivity of the RIM systems, but that they also affect the mechanical properties of the resulting molded articles, so that molded articles having certain three-dimensional shapes, for example, could not be produced or could only be produced with reduced mechanical properties, since the required reactive polyurethane mixtures were not sufficiently free-flowing.